1. Field of the Invention
The present invention relates to an aluminum alloy for super molding, which is subjected to super plastic processing in a temperature range of, for example, from 350 to 560.degree. C., and to a production method for the same.
2. Description of the Related Art
Various materials have so far been investigated which are stretched to a notably large extent without causing local deformation such as necking when they are heated to a prescribed temperature and pulled. In recent years, with respect to an aluminum alloy, super plastic materials which are elongated by 150% or more at, for example, 350.degree. C. or higher are investigated.
Known as a conventional aluminum series super plastic materials are, for example, an Al-78% Zn alloy, an Al-33% Cn alloy, an Al-6% Cu-0.4% Zr alloy, (SUPRALL), an Al--Zn--Mg--Cu alloy (7475 alloy and 7077 alloy of AA standard), and an Al-2.5 to 6.0% Mg-0.05 to 0.6% Zr alloy. Molding processing of a complicated form can readily be made with such super plastic materials.
With respect to JIS No. 5000 series alloys such as an Al--Mg series alloy, it has been confirmed by the present inventors that not only the Al-2.5 to 6.0% Mg-0.05 to 0.6% Zr alloy described above but also other alloys can be used as super plastic molding materials of a so-called static recrystallized type by adequately controlling the production process and adjusting the recrystallization grain size in super plastic molding so that it becomes markedly fine as well as by adequately adjusting the component composition. These are disclosed in Japanese patent application No. 5-47431.
Meanwhile, plastic materials of the discussed type are considered to be applicable to various fields since they provide excellent molding performance at prescribed temperatures. Also with respect to aluminum series super plastic materials, they are considered to be applicable to those fields requiring complicated forms as various structural materials for, for example, automobiles and vehicles such as streetcars. In the case where they are used as structural materials as described above, requirements not only from the viewpoint of facility in molding but also in terms of strength have to be sufficiently considered.
However, conventional aluminum series super plastic molding materials involve the following problems. That is, they can be molded to complicated forms but in the case where they are locally stretched to a large extent, the plate thickness of this stretched part becomes too thin, which causes a deficiency in the structural strength, and they can not be used as structural materials.
Accordingly, it is considered that preliminary molding (hereinafter referred to as pre-molding) is given in advance by cold press molding prior to super plastic molding to make a rough form and then, molding to a complicated form is carried out by the super plastic molding. Local stretching and thinning generated in the super plastic molding can be avoided by such molding. However, it is problematic to perform cold pre-molding before the super plastic molding because such pre-molding lowers the super plastic characteristic to a large extent or the pre-molding itself becomes very difficult to perform with rolled plates of Al--Mg series super plastic molding aluminum alloys of the conventional static recrystallized type as described above.
That is, in the case where the rolled plate of the Al--Mg series super plastic molding aluminum alloy of the conventional static recrystallized type is subjected to super plastic molding, two methods are generally available as described below. The first one is a method in which a plate after rolling is subjected to recrystallization processing and then to super plastic molding at a prescribed super plastic temperature range. The second one is a method in which a rolled plate is put in an oven to complete recrystallization while heating it up to a super plastic molding temperature.
In the first method described above, while a soft plate having recrystallized crystals is subjected to pre-molding, the pre-molding itself is easy, but cold distortion is caused during the pre-molding, and crystal particles are partially coarsened at a super plastic temperature to largely reduce the super plastic molding characteristic.
Meanwhile, in the second method described above, since a plate prior to being recrystallized is subjected to cold pre-molding, the bending performance of the plate is poor in the pre-molding, and the cold pre-molding is usually difficult, which makes even simple bend molding impossible.
The present invention was completed to overcome the above problems of known super plastic materials and methods of molding same. The present invention is made to provide an Al--Mg series aluminum alloy for super plastic molding which has made cold pre-molding actually possible without damaging the super plastic characteristic.
The present inventors repeated various experiments and investigations on the Al--Mg series aluminum alloy for super plastic molding. The results thereof have led to the finding that appropriately adjusting the component composition of the alloy and properly setting and adjusting production conditions allows a crystalline structure to comprise a non-recrystallized structure, a 90.degree. bending radius to become 7.5 times (hereinafter referred to as 7.5 t) or less of a plate thickness and a yield strength ratio before and after annealing (yield strength after annealing/yield strength before annealing) to be set to 70% or more, whereby the problems described above can be solved.
The present invention has been completed based on such knowledge.